[Elsie]

You never know when an angel will knock on your door.

And, by just the knock, you can tell it's an ANGEL!

(If it were a DEMON the sound would be more like a squish-squish or thud-thud, because of lack of bones to make a good sounding KNOCK.)

[Monkey Face]

Well, that would be MY guess...

I suspect a demon would stand outside the door and whine and roar because it can’t knock, but wants someone to come out and “play.”